Hillyah was a female harvest mouse, wife to Oreal and mother of twins, Irgle and Ralg. Her family moved into the Redwall Abbey Gatehouse to serve as Gatekeepers half a season after the death of Old Gruggle. She assisted Brinty, Girry, Tribsy, Old Quelt and Sister Snowdrop on their riddle quest regarding Green Isle and the High Rhulain when she figured out that T.O.A.L. stood for a book titled Tales of Ancient Life (a book written by Sister Geminya under the name Minegay). She recalled that she had seen it holding up one corner of her bed. Later, Hillyah and Oreal had to defend their twins from a savage gannet that had been wounded and residing in the Abbey attic.